Subject: Re: [patch 1/3] powerpc: Move 64bit VDSO to improve context switch performance

From: Andreas Schwab <[email-masked]>
Date: Sat, 03 Oct 2009 16:51:29 +0200

Andreas Schwab <[email-masked]> writes:

> Andreas Schwab <[email-masked]> writes:
>
>> Anton Blanchard <[email-masked]> writes:
>>
>>> On 64bit applications the VDSO is the only thing in segment 0. Since the VDSO
>>> is position independent we can remove the hint and let get_unmapped_area pick
>>> an area.
>>
>> This breaks gdb.  The section table in the VDSO image when mapped into
>> the process no longer contains meaningful values, and gdb rejects it.
>
> The problem is that the load segment requires 64k alignment, but the
> page allocater of course only provides PAGE_SIZE alignment, causing the
> image to be unaligned in memory.

Here is a patch.  The disadvantage is that now gdb complains that the
vdso does not contain section headers, because they are no longer
covered by the load segment.

Andreas.
---
>>From 003c323c753d8717e58220c9560329882bcfa2c2 Mon Sep 17 00:00:00 2001
From: Andreas Schwab <[email-masked]>
Date: Sat, 3 Oct 2009 16:46:45 +0200
Subject: [PATCH] powerpc: fix unaligned load segment in vdso64

Since the page allocator can only guarantee PAGE_SIZE alignment the load
segment in the vdso object cannot claim more than that alignment.  Use 4k
alignment which is the minimum possible page size.

Signed-off-by: Andreas Schwab <[email-masked]>
---
 arch/powerpc/kernel/vdso64/Makefile |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/arch/powerpc/kernel/vdso64/Makefile b/arch/powerpc/kernel/vdso64/Makefile
index 79da65d..d4be303 100644
--- a/arch/powerpc/kernel/vdso64/Makefile
+++ b/arch/powerpc/kernel/vdso64/Makefile
@@ -10,7 +10,9 @@ obj-vdso64 := $(addprefix $(obj)/, $(obj-vdso64))
 GCOV_PROFILE := n
 
 EXTRA_CFLAGS := -shared -fno-common -fno-builtin
+# Force maximum page size of 4k so that it works with any page size
 EXTRA_CFLAGS += -nostdlib -Wl,-soname=linux-vdso64.so.1 \
+		-Wl,-z,max-page-size=4096 \
 		$(call cc-ldoption, -Wl$(comma)--hash-style=sysv)
 EXTRA_AFLAGS := -D__VDSO64__ -s
 
-- 
1.6.4.4

-- 
Andreas Schwab, [email-masked]
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